In recent years, a giant magnetoresistance (GMR) effect device consisting of a multi-layered film of ferromagnetic layer/nonmagnetic metal layer and a ferromagnetic tunneling junction (MTJ) device comprised of ferromagnetic layer/insulating layer/ferromagnetic layer have been drawing attention as new magnetic field sensors and nonvolatile random access magnetic memory (MRAM) devices. As giant magnetoresistance effect devices there are known those of CIP (Current in Plane) structure in which an electric current is passed in a film plane and those of CPP (Current Perpendicular to the Plane) in which a current is passed in a direction perpendicular to a film plane. Principles of the giant magnetoresistance effect device reside in spin dependent scattering at an interface between a magnetic and a nonmagnetic layer and also in contribution of spin dependent scattering in a magnetic material (bulk scattering). Therefore, in general, the giant magnetoresistance effect device of the CPP structure that is effective in enhancement of the bulk scattering has larger GMR than that of the giant magnetoresistance effect device of CIP structure.
As such giant magnetoresistance effect devices, those of spin valve type in which an antiferromagnetic layer is brought close to one of ferromagnetic layers to pin spin of the ferromagnetic layer have been used. In the case of spin valve type giant magnetoresistance effect devices, since the resistivity of the antiferromagnetic layer is about 200 μΩ·cm or so that is larger by two orders than that of the GMR film, their GMR effect have been weakened. The value of magnetoresistance of the giant magnetoresistance effect device of spin valve type and with CPP structure is as small as 1% or lower. Thus, while giant magnetoresistance effect device of CIP structure have already been practically used in the read head of a hard disk, no giant magnetoresistance effect device of CPP structure has as yet been brought to practical applications. However, if a magnetic material such as a half-metal that has a large spin polarization is developed, its use is expected to bring about large bulk scattering with the result of a large CPP-GMR anticipated.
On the other hand, with a MTJ device in which two ferromagnetic layers are controllably magnetized by an external magnetic field to magnetically orient parallel or antiparallel to each other to obtain at room temperature what is called the tunneling magnetoresistance (TMR) effect in which tunnel currents in directions perpendicular to film plane are different in magnitude from each other (see Non-patent Reference 1). It is known that the TMR effect depends on spin polarization P at an interface between a ferromagnetic and an insulating material used. With the assumption that the spin polarization values of two ferromagnetic materials are P1 and P2, respectively, it is known that TMR is given by equation (1) (Jullier's equation) below.TMR=2P1P2/(1−P1P2)  (1)where spin polarization P of a ferromagnetic material takes a value: 0<P≦1.
If Al oxide film is used as the insulating material constituting a barrier, the maximum value of TMR at room temperature which has been obtained at present is about 60% of that when CoFeB alloy is used.
MTJ devices at present have been put to practical use in magnetic heads for hard disks and in the future are expected of their application to a nonvolatile magnetic random access memory (MRAM). In the MRAM, MTJ devices are arranged in a matrix. A magnetic field is applied to them by flowing an electric current to separately provided wirings so that two magnetic layers constituting each MTJ device are controlled parallel or antiparallel to each other to record “1” or “0”. Its readout is performed by utilizing the TMR effect. However, reducing the devices in size to increase the density of the MRAM causes noises to grow due to their non-uniformity, giving rise to the problem that the TMR value is deficient at present. Thus, the need arises to develop a device that exhibits a larger TMR.
As is apparent from equation (1), the use of a magnetic material having P=1 allows expectation that the TMR is infinitely large. A magnetic material satisfying P=1 is termed as a half-metal. So far, from band structure computations, oxides such as Fe3O4, CrO2, (La—Sr)MnO3, Th2MnO7 and Sr2FeMoO6, half-Heusler alloys such as NiMnSb and full-Heusler alloys such as Co2MnGe, Co2MnSi and Co2CrAl having L21 structure are known as half-metals.
Of late, a large TMR of 200% or more at room temperature has been attained using MgO barrier and a ferromagnetic layer of Fe or FeCoB. However, it was utilizing the MgO barrier and special band structures of the above ferromagnetic layer. Such a large TMR was attained only with their particular combination. The spin polarization of the ferromagnetic layer itself is not so large. Indeed, the spin polarization of Fe is around 0.4 and that of FeCoB is about 0.6. Such a large TMR can not be obtained by using Al oxide barrier.
In order to have the L21 structure with a full-Heusler alloy, it is usually necessary that a substrate be heated to 300° C. or higher, or, after deposition at room temperature, be heat-treated at 300° C. or higher. However, even if the L21 structure is obtained, there has been no report that a prepared thin film is recognized as a half-metal at room temperature. In fact, any of the tunnel junction devices prepared using such a half-metal materials had unexpectedly low TMRs at room temperature. When Al oxide film was used as a barrier, they were 60 to 70% at the maximum of those of the cases where Co2MnAl and Co2MnSi Heusler alloys were used. Moreover, these Heusler alloys containing Mn are liable to be oxidized at an interface and hardly to have a stabilized TMR. Further, due to their liability to oxidation they are large in junction resistance and commonly have a product of resistance and area (RA) amounting to 107Ω·μm2 or more. Too high the resistance makes the application to hard disk and mass-storage MRAM difficult.
In practice, it is very difficult to fabricate a thin film of such a half-metal. The causes are considered to include the susceptibility of property of a half-metal to its composition and regularity of its atomic arrangement, especially in a tunnel junction, the difficulty to have the electronic state of a half-metal at its interface, and also the increase in surface roughness and the interface oxidation caused by heating or heat-treating a substrate as necessitated in securing the structure of a half-metal thin film.
The present inventors had fabricated MTJ devices using various full-Heusler alloys in the past. We have reported that when a Co2FeAl full-Heusler alloy thin film fabricated on a MgO substrate is used, a TMR of 50% or more at room temperature is obtained stably (see Non-patent Reference 2). It has also discovered that the Co2FeAl structure then is not of L21 but B2 in disordered structure and it is difficult to obtain the L21 structure in this composition.
Meanwhile, it has lately been reported that Co2FeSi full-Heusler alloy becomes a half-metal. It has been reported by the present inventors that this material has the L21 structure easily obtained in bulk and the L21 structure obtained in a thin film as well. However, it is reported by the present inventors in Non-patent Reference 3 that in a tunnel junction using this material, the TMR at room temperature is as low as around 40% and no large TMR as expected from the half-metal is obtained.    Non-patent Reference 1: T. Miyazaki and N. Tezuka “Spin polarized tunneling in ferromagnet/insulator/ferromagnet junctions”, J. Magn. Magn. Mater. 151, pp. 403-410, 1995;    Non-patent Reference 2: Okamura et al., Appl. Phys. Lett., Vol. 86, pp. 232503-1 to 232503-3, 2005; and    Non-patent Reference 3: Inomata et al., J. Phys. D, Vol. 39, pp. 816-823, 2006